I own a P290. My experience with the Nano and Shield has only been holding and dry firing one in a shop. I will not speak to those two guns as I do not believe that some shop time with one qualifies me to say much other than that I may consider owning a Shield down the road, or at least shooting one to see what it's really all about. I will say that overall it seemd to me a good bit larger than the P290. I haven't compared specs so that is just my impression based on limited exposure with the Shield.
As to the P290, I like it quite a bit. Mine, with a build date of March 2011, had to go back to SIG. After my first range session the hammer would not go down all the way upon pulling the trigger. It would stop short of the firing pin. There were no other issues with the gun. After a trip to SIG, with a turn-around time of eight days door to door, the gun came back fixed. The paperwork indicated an upgraded trigger as well as a new mag release. There had been no previous issues with the mag release. The gun now performs perfectly. While I wish SIG would have gotten this pistol right the first time, their customer service really came through. The trigger is also noticeably lighter. This upgrade was prior to the RS reset version upgrade, and I am happy with the gun as it. Re-strike isn't important to me as my practice is tap/rack. But, the trigger upgrade they did has resulted in a much lighter trigger. The old trigger was smooth (not at all gritty) but heavy. The new trigger is much lighter and any stacking seems to be dead minimal. For my purposes SIG has got this one right.
Overall I find the P290 to be light shooting for its size. Accuracy with about 200 rounds has been good and is getting better. At eleven yards all rounds are into five inches, which is just fine for the purposes of this gun. I can tell that accuracy will improve with practice but I find it to be acceptable at this point. Function has been 100%. The (SigLite night sights) sights are actually outstanding for this size gun. I believe they are the same size sights used on a SIG like the P220. While the sights are a big plus they will become a bit less so as I will be doing a lot of point shooting. The gun shoots better (more accurate and controllable) with the extended eight round mag which is my back-up. I see no use for a laser.
As to carrying the gun, I have limited it to pocket carry. The gun gets a bad rap as being thick, but in terms of height and length it is actually quite compact. In a Galco Nemesis it is quickly forgotten in my pocket. It's heavier than many pocket guns, but carried the right way it ceases to be an issue for me.
Overall, after the initial hiccup, the gun is a keeper. Stick to guns with newer build dates and you will be good to go,